1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine having an exhaust-gas purification catalyst unit disposed in an exhaust gas passage. In particular, the invention relates to such an air-fuel ratio control apparatus and an air-fuel ratio control method that control the fuel supply amount based on the output value of an air-fuel ratio sensor such that the air-fuel ratio of exhaust gas flowing into the exhaust-gas purification catalyst unit equals a target air-fuel ratio.
2. Description of the Related Art
Three-way catalyst units are often used in internal combustion engines of vehicles. A three-way catalyst unit purifies exhaust gas by facilitating the oxidization reactions of HC (hydrocarbon) and CO (carbon monoxide) that are unburned components and by facilitating the reduction reactions of NOx that are produced through reactions between nitrogen in air and oxygen left unburned in exhaust gas. To efficiently use such oxidization and reduction performances of a three-way catalyst unit, the air-fuel ratio of the internal combustion engine, which represents the combustion state of the internal combustion engine, needs to be maintained near the stoicheiometric air-fuel ratio. Thus, for fuel injection control in an internal combustion engine, typically, an oxygen sensor (oxygen concentration sensor) is provided which detects whether the air-fuel ratio of exhaust gas is rich or lean, that is, lower or higher than the stoicheiometric air-fuel ratio based on the concentration of oxygen left in exhaust gas, and air-fuel ratio feedback control that corrects the fuel supply amount based on the output of the oxygen sensor is executed.
For example, Japanese Patent Applications No. 11-82114 (JP-A-11-82114) and No. 2007-107512 (JP-A-2007-107512) describe double-oxygen-sensor systems for air-fuel ratio feedback control. According to these systems, an oxygen sensor for detecting the oxygen concentration in exhaust gas is arranged upstream of a three-way catalyst unit so as to be as close to the combustion chambers of the internal combustion engine as possible, and another oxygen sensor is provided downstream of the three-way catalyst unit. The downstream oxygen sensor is used to compensate for the variation of the output characteristic of the upstream oxygen sensor. That is, as the exhaust gas discharged from the internal combustion engine flows to the downstream side of the three-way catalyst unit, the exhaust gas is agitated and the oxygen concentration in the entire exhaust gas becomes substantially uniform due to the catalytic effects of the three-way catalyst unit. Therefore, the output of the downstream oxygen sensor changes more gently than the output of the upstream oxygen sensor does, and thus the downstream oxygen sensor can more reliably detect whether the air-fuel ratio of the entire air-fuel mixture is rich or lean. In such a double-oxygen-sensor system, sub-air-fuel ratio feedback control is executed using the downstream oxygen sensor, as well as the main air-fuel ratio feedback control that is executed using the upstream oxygen sensor. For example, a constant related to the main air-fuel ratio feedback control is corrected based on the output of the downstream oxygen sensor so as to compensate for the variation of the output characteristic of the upstream oxygen sensor and thus improve the accuracy of the air-fuel ratio control.
Further, in recent years, an internal combustion engine has been developed which incorporates a three-way catalyst unit having an oxygen storage capability and controls the air-fuel ratio flowing into the three-way catalyst unit such that the purification performance of the three-way catalyst unit can be utilized in a stable manner. Having an oxygen storage capability, a three-way catalyst unit stores surplus oxygen when the air-fuel ratio of exhaust gas is lean (higher than the stoicheiometric air-fuel ratio) and releases the oxygen stored in the three-way catalyst unit when the air-fuel ratio of exhaust gas is rich (lower than the stoicheiometric air-fuel ratio), and such an oxygen storage capability of a three-way catalyst unit is limited. As such, in order to efficiently utilize the oxygen storage capability of a three-way catalyst unit, it is important to maintain the amount of oxygen stored in the three-way catalyst unit at a predetermined amount, for example, at a half of the maximum oxygen storage capacity of the three-way catalyst unit. In this case, the three-way catalyst unit can always provide constant oxygen storage and release effects despite whether the air-fuel ratio of exhaust gas is rich or lean, enabling continuous use of constant oxidization and reduction effects of the three-way catalyst unit.
For example, an air-fuel ratio control apparatus is known which is incorporated in an internal combustion engine in which the amount of oxygen stored in a three-way catalyst unit is controlled to a desired level in order to maintain a desired level of purification performance of the three-way catalyst unit. This air-fuel ratio control apparatus incorporates two air-fuel ratio sensors provided upstream and downstream of the three-way catalyst unit, respectively. More specifically, the air-fuel ratio sensor provided upstream of the three-way catalyst unit is a linear air-fuel ratio sensor that linearly detects the air-fuel ratio of exhaust gas, and the air-fuel ratio sensor provided downstream of the three-way catalyst unit is an oxygen sensor that outputs voltage that varies depending upon whether the air-fuel ratio of exhaust gas is rich or lean, that is, whether it is lower or higher than the stoicheiometric air-fuel ratio. According to this air-fuel ratio control apparatus, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit is detected using the linear air-fuel ratio sensor that is provided upstream of the three-way catalyst unit, and the state of the air-fuel ratio of the atmosphere in the three-way catalyst unit is detected by the oxygen sensor that is provided downstream of the three-way catalyst unit, and the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit is controlled based on the output of the oxygen sensor, and feedback control of the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is executed based on the output of the linear air-fuel ratio sensor such that said air-fuel ratio equals the target air-fuel ratio.
In the case of an air-fuel ratio control apparatus that maintains the oxygen amount in the three-way catalyst at a constant level by executing feedback control of the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit based on the output of the oxygen sensor and controls the air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit to the target air-fuel ratio by executing feedback control of the fuel injection amount based on the output of the linear air-fuel ratio sensor, in a state where the intake air amount is large, such when an acceleration operation is being performed, (will hereinafter be referred to as “large intake-air-amount state” where necessary), the amount of oxygen stored in the three-way catalyst tends to be corrected by a large amount, and this may cause the air-fuel ratio of the atmosphere in the three-way catalyst to largely deviate from a target air-fuel ratio range near the stoicheiometric air-fuel ratio where the three components in exhaust gas, that is, HC, CO, and NOx can be all removed at removal rates of 80% or more. The target air-fuel ratio range will hereinafter be referred to as “purification window” where necessary.
In the case of an air-fuel ratio control apparatus that maintains the oxygen amount in the three-way catalyst at a constant level by executing feedback control of the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit based on the output of the oxygen sensor and controls the air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit to the target air-fuel ratio by executing feedback control of the fuel injection amount based on the output of the linear air-fuel ratio sensor, even if the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst is kept unchanged, the degree of oxygen adsorption to the three-way catalyst and the degree of oxygen release from the three-way catalyst vary depending upon the intake air amount. For example, in a case where the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit is controlled to a lean value, that is, a value higher than the stoicheiometric air-fuel ratio, the larger the intake air amount, the larger the amount of oxygen stored to the three-way catalyst unit per unit time, and therefore the sooner it reaches the maximum oxygen storage capacity of the three-way catalyst unit. As such, even in a case where the target air-fuel ratio of the exhaust gas flowing into the three-way catalyst unit is maintained at a constant value, the larger the intake air amount, the larger the amount of oxygen stored to the three-way catalyst unit per unit time, and thus the larger the correction amount for the oxygen amount in the three-way catalyst unit, increasing the possibility of the air-fuel ratio of the atmosphere in the three-way catalyst unit largely deviating from the purification window.
However, such deviation of the air-fuel ratio of the atmosphere in an exhaust-gas purification catalyst unit having an oxygen storage capacity (e.g., three-way catalyst) from its purification window can be prevented and thus a resultant increase in the emissions of the internal combustion engine can be prevented by, for example, maintaining the correction amount per unit time of the oxygen amount in the exhaust-gas purification catalyst unit at a constant level, that is, by maintaining the amount of oxygen stored to or released from the exhaust-gas purification catalyst unit per unit time at a constant level.
To accomplish the above control, for example, in a case where feedback control of the target air-fuel ratio of the exhaust gas flowing into the exhaust-gas purification catalyst unit is executed based on the output of an oxygen sensor provided downstream of the exhaust-gas purification catalyst unit and the intake air amount, PI control (Proportional-Integral control) is executed as said feedback control. In this PI control, in order to maintain the correction amount per unit time of the oxygen amount in the exhaust-gas purification catalyst unit at a constant level, the proportional correction term for the PI control is multiplied by a first correction coefficient that is made smaller the larger the intake air amount, and the integral correction term for the PI control is multiplied by a second correction coefficient that is made larger the larger the intake air amount.
However, if such control is executed in a state where the intake air amount is extremely small, like during idling of the internal combustion engine, and the air-fuel ratio detected by the oxygen sensor provided downstream of the exhaust-gas purification catalyst unit is rich, NOx may not be sufficiently removed through reduction reactions at the exhaust-gas purification catalyst unit. During the control described above, if the intake air amount is extremely small and the air-fuel ratio detected by the oxygen sensor is rich, the first correction coefficient that is multiplied by the proportional correction term for the PI control is made larger than it is when the intake air amount is large, whereby the target air-fuel ratio of the exhaust gas flowing into the exhaust-gas purification catalyst unit is increased to a lean value. At this time, however, if an operation that causes a sharp increase in the intake air amount, such as a rapid acceleration operation, is performed, the air-fuel ratio of the atmosphere in the exhaust-gas purification catalyst unit sharply increases, and this may result in the exhaust-gas purification catalyst unit failing to sufficiently remove NOx in exhaust gas through reduction reactions.